The present invention relates to a boiling water type nuclear reactor and, more particularly, to a core of this type of reactor.
The average ratio of weight of vapor to the total weight of the coolant (Referred to as void quality hereinafter) in the core of boiling water reactor during operation is usually 13 to 15%. The thermal condition of operation of the boiling water reactors is limited by various factors, typically including the maximum linear heat generating rate and the minimum critical power ratio (MCPR). According to the current method of design, the size of the reactor core is determined for a given power, such that the maximum linear heat generating rate is maintained at a level lower than 13.4 kw/ft. Then, the flow rate of the coolant flowing through the core is determined so as to provide such a value of the minimum critical power ratio as not to incur the burning out of the fuel rods. A void quality of 13 to 15% at the core outlet is automatically derived from this method of design and has been adopted in existing boiling water reactors.